1. Field of the Invention
The present invention relates to a circuit using thin-film semiconductor devices having crystallinity.
2. Description of the Related Art
There has been known such a semiconductor device that a thin silicon film is formed on a glass substrate or a quartz substrate and the silicon film is used. In recent years, there has been developed a technique in which a silicon film is crystallized by heat or laser so that the device using the crystallized silicon film can be operated at high speed.
In a semiconductor device of polycrystalline silicon crystallized by heat, a heat treatment at a relatively high temperature such as 800.degree. C., 900.degree. C. or more is used. Thus, a quartz substrate which can withstand the above high temperature is used as a substrate.
In the technique of using laser for crystallization, thermal damage to a substrate hardly occurs, so that a glass substrate can be used.
In the present circumstances, TFTs are used to integrate an active matrix circuit and a peripheral drive circuit of an active matrix type liquid crystal display device on the same substrate. That is, it is practiced that the active matrix circuit and the peripheral drive circuit are formed on the same substrate by using the TFTs.
Also, in addition to the peripheral drive circuit (constituted by a shift resistor circuit, a buffer circuit, or the like), it has been considered to integrate an oscillation circuit, a D/A converter, an A/D converter, and a digital circuit for various kinds of image processing on the same substrate.
In the case where such various circuits are constituted, there is required a circuit structure in which the characteristics of paired thin-film transistors must strictly coincide with each other, such as a differential circuit.
Also, in a CMOS circuit (circuit in which a P-channel type thin-film transistor and an N-channel type thin-film transistor are complementarily constituted), it is preferable that the characteristic of the P-channel type thin-film transistor can possibly coincide with that of the N-channel type thin-film transistor.
However, if the P-channel type thin-film transistor and the N-channel type thin-film transistor are fabricated with the same structure and dimensions, the mobility of the N-channel type thin-film transistor is larger than the P-channel type thin-film transistor by about 30 to 50%.
Accordingly, when the CMOS circuit is operated at high frequency, there occurs an event that the balance of operation is lost between the P-channel type thin-film transistor and the N-channel type thin-film transistor.